The cell surface receptors for IgE and C3 will be characterized. Rat immunoglobulins other than IgE and the complement anaphylactoxins will be examined for interaction with the IgE receptor in order to examine its specificity. Radiolabeled diisopropylfluorophosphate will be used to tag the serine esterase that is activated in IGE-mediated reactions and the labeled esterase will be examined for possible identity with the IgE receptor. This receptor will be specifically isolated by physiochemical procedures, avoiding any interaction with IgE. The isolated material will be examined for proteolytic activity and, by incorporating it into liposomes, for ability to influence the calcium flux across model membranes. Inside-out plasma membrane vesicles will be prepared and analyzed to determine if the IgE receptor is a transmembrane protein. The interaction of human C3 and the fragments C3b and C3d with cultured lymphoblastoid cells and with normal peripheral blood monocytes and neutrophils will be studied. The affinity of binding and the number of binding sites will be compared. C3 receptor-bearing cells will be surface labeled and the receptors solubilized by either mechanical or detergent treatments; the receptors will then be specifically immunoprecipitated and analyzed by SDS-polyacrylamide gel electrophoresis. Possible differences in C3 receptors bound to normal vs. transformed cells will be examined. Normal cells will be treated with neuraminidase to attempt to render the C3b receptors on the normal cells "transformed" in the sense that the cells will then be susceptible to alternative complement pathway induced lysis. The effect of the control proteins, beta 1 H and C3b Inactivator, on prevention of this lysis will be examined.